1. Field of the Invention
The present invention relates to a method and apparatus for modulating light using a semiconductor element having a quantum well structure at a very high speed on the order of picoseconds or less.
2. Related Background Art
A device utilizing a photorefractive (PR) effect is proposed as a conventional optical modulation apparatus. This PR effect will be described with reference to FIG. 1.
FIG. 1 is a diagram showing an energy band of a bulk electrooptical crystal. When an electron 41 located in a given impurity level 42 is optically excited and moved to a conduction band, this electron 41 is diffused and drifted in the conduction band and then captured by a capture level 43. A spatial electric field formed by this electron capture is subjected to a nonlinear change in refractive index by an electrooptical effect (i.e., a Pockels effect). This phenomenon is called the PR effect. The impurity level 42 is generally called a deep level having an energy gap sufficiently larger than that of thermal energy (26 meV) at room temperature with respect to a donor or acceptor level serving as a shallow level. The capture level 43 generally consists of an ionization level of the impurity level 42.
The PR effect has an advantage in that a nonlinear change in refractive index is obtained with a laser beam power of about several mW. However, the response speed of this change is a maximum of several microseconds because movement and capture of an optically excited carriers (i.e, electrons in this case) are utilized.
In order to increase the response speed, a semiconductor well structure having a barrier layer of a graded gap structure whose band gap is changed in the direction of the thickness of the layer is proposed in Physical Review Letters. Ralph et. al., Vol. 63, pp. 2272-2275 (1989). This semiconductor well structure is shown in FIGS. 2A to 2C. As shown in FIG. 2A, valence and conduction bands 51 and 52 of a barrier layer have graded gap structures, so that a built-in electric field is generated by these structures. In the built-in electric field, holes 53 and electrons 54 formed upon incidence of light on the graded gap barrier layer are drifted by the built-in electric field and are respectively captured by quantum wells 55 and 56 in the valence and conduction bands. At this time, the electrons 54 are captured earlier than the holes 53 by the quantum wells 56 due to a difference between drift speeds of the electrons 54 and the holes 53, as shown in FIG. 2B. For this reason, as shown in FIG. 2C, until the holes 53 are captured in the quantum wells 55, a transient spatial electric field is formed between the electrons 54 and the holes 53. This electric field induces a change in refractive index through an electrooptical effect in the barrier layer.
After both the electrons 54 and the holes 53 are captured by the quantum wells 56 and 55, no spatial separation is present between the electrons 54 and the holes 53. The above transient spatial electric field then disappears. The response speed of the PR effect in the above transient process is mainly determined by the drift time for which the electrons 54 are captured by the quantum wells 56. Therefore, a rise response speed can be experimentally confirmed to be on the order of about picoseconds.
A fall time is mainly determined by time required for capturing the holes 53 in the quantum wells 55 and is not limited by a recombination time of electrons 54 and the holes 53 because no spatial separation occurs between the electrons 54 and the holes 53, both of which are respectively captured in the quantum wells 56 and 55, and no spatial electric field is generated. Therefore, the recombination time does not contribute to the PR effect. Therefore, the fall time can be expected to obtain the range of the order of picoseconds to the order of 10 picoseconds. In practice, however, a tailing phenomenon of fall time on the order of nanoseconds in the PR effect caused by impurity trapping in the barrier layer undesirably occurs.
As described above, in pure optical modulation of a light wave, rise and fall response times are limited by running time of the optically excited carriers and capture in the impurity level even in actual carrier excitation using a quantum well structure.